A known example of a lock device of this type is a technique disclosed in Patent Document 1. In this technique, a striker is provided on a vehicle body side, and a lock mechanism is provided on a seat side. The lock mechanism includes a base plate having a recess capable of receiving the striker, a hook, a pawl and a cam. The hook is rotatably attached to the base plate via a shaft, so that the lock mechanism is in a locked condition in which the striker is held between the hook and the recess of the base plate or in an unlocked condition in which the hook is spaced away from the striker. The pawl and the cam are rotatably attached to the base plate via a common shaft. The pawl is held by an elastic force of a lock spring at a rotating position in which the pawl can engage the hook in the locked condition. The cam imparts a pressing force (a pushing force) in a locking direction to the hook in the locked condition by an elastic force of a push spring. As a result, a clearance between the striker and the lock mechanism in the locked condition is forcibly reduced.
FIG. 7 is a schematic view of a lock device having substantially the same function as the technique disclosed in Patent Document 1. The drawing shows the lock device that is in an unlocked condition. A hook 126 of a lock mechanism 120 is held at a rotating position in which it is spaced apart from a striker 150. In this condition, both a pressing force due to a lock spring 134 and a pressing force due to a push spring 142 acting on a pawl 130 through a cam 140 are applied to a contact point C of the hook 126 and the pawl 130. As a result of these pressing forces, at the contact point C, the hook 126 is applied with a force F in a direction of a normal with respect to the arc of the hook 126 and the pawl 130, so that the lock mechanism 120 is maintained in the unlocked condition.
Patent Document 1: International Publication No. 2004/069585